Needleless or jet injectors typically use pressurized gas to accelerate a plunger into an ampule holding liquid injectant, thereby causing the injectant to flow out of the ampule with sufficient pressure or velocity to penetrate the skin of the patient. Ampules used with needleless injectors are preferably disposable, must be strong enough to withstand the stresses of injection, and must be made from a biocompatible material. In addition, ampules are preferably transparent to allow for visual inspection of their contents.
Thermoplastic injection molding is preferred for manufacturing ampules because there are numerous thermoplastics available which are biocompatible, transparent, strong, and have good molding characteristics. In addition, in a typical injection molding process for manufacturing ampules, an injection cycle can be completed in less than one minute, and multiple molds or cavities allow for proportionally lower costs, a significant factor for disposable ampules. The injection molding process also can achieve smooth contours and transitions within the ampule, which provides for more efficient liquid injectant flow during injection. Sufficient manufacturing tolerances can also be economically maintained with thermoplastic injection molding, whereas attempting to achieve the same result with machining or drilling would be difficult. Thermoplastic injection molding also results in ampules free of particulate matter, whereas machining and drilling processes create particles that must be removed in a separate process which increases cost and complexity.
Molds used to produce ampules are generally constructed of two primary elements, a cavity housing and a core pin. The cavity housing forms the external surfaces of the ampule, while the core pin forms the internal surfaces, including the critical nozzle section.
Frequently, in thermoplastic injection molding production of ampules, the core pin tip fractures or breaks during the molding process. Bending stresses on the core pin tip can be introduced when the tip of the core pin is inserted into a pilot hole, as the mold is closed. Bending of the core pin tip also occurs when the hot liquid plastic is injected into the mold. The pressure of flowing plastic is not uniform on all sides of the pin, resulting in deformation of the core pin tip. This produces bending fatigue of the tip and eventual breakage.
Breakage also occurs when the mold is opened and the ampule is removed after the molten plastic has solidified. The core pin tip becomes very hot during the injection of the plastic because it is not cooled by water as are other surf aces of the mold. This can result in welding of the plastic to the steel pin tip. Although the preferred ampule molding material, high strength polycarbonate, has good clarity and strength, it unfortunately also sticks to the mold surfaces more than most other plastics.
The force required to shear a welded plastic ampule from a core pin is proportional to the diameter of the pin, while the strength of the pin is proportional to the diameter squared. The stress on the pin during removal of the ampule f rom the mold is therefore inversely proportional to the diameter of the pin. Hence, there is a limit on how small the core pin tip can be before it cannot be withdrawn f rom the part without breaking, and with smaller diameter core pin tips, the breakage rate is higher. The rate of breakage of the core pin tip is inversely related to the diameter of the ampule nozzle. For ampules having 0.008 inch diameter nozzles, the breakage rate is low and the molding process is reliable enough to produce parts on an ongoing basis. On the other hand, for ampules with 0.004 inch diameter nozzles, the breakage rate is very high and makes the injection molding process not cost effective because the process must be stopped and the mold disassembled and repaired with each core pin tip breakage.
The pilot hole typically has a 0.0015 inch clearance gap around the core pin tip. This very close fit prevents flashing of the plastic into the clearance gap. However, if the core pin tip strikes the edge of the pilot hole during closure of the mold, the tip will bend and/or break. The dimensional control of the pilot hole in the cavity housing-core pin registration in a multiple cavity tool, with thermal distortion, approaches the limits of the current art in mold making and injection molding. Accordingly, there is a need for an improved method for manufacturing ampules.